Get 404 page not found when browsing a web service you know is there.
For example, from IIS you right-click and select “Browse” on the .svc file (so you know it is really there). NOTE: the 404 is a “red herring” an outright lie – a bad error given to us by Microsoft to confuse us.

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Try checking to see if you have ASP.NET 1.1 instead of 2.0 set.
Right click your “virtual directory”, then click the “ASP.NET” tab.
.NET 2.0 and 3.5 use the same “CLR”, therefore you will see 2.0.50727 in the
ASP.NET version on this tab. If you see 1.x, then change it to 2.0.50727.

From IIS Admin, right-click the project, …

If and when you get this error on Windows 2003, there is an additional setup required.
Click “Web Service Extensions” at the bottom of IIS, and check the “ASP.NET V2.0.50727” and see if is is “allowed” instead of “prohibited”.

If you get “service unavailable”, then make sure the userid that is running your application pool (Win 2003/IIS 6) is a member of the “IIS_WPG” (IIS worker process group) and that the password is correct.